As having good corrosion resistance, stainless steel containing Cr and/or Ni now has many increasing applications. However, since stainless steel is a high alloy containing a large amount of expensive elements such as Cr and Ni and since it is produced in a complicated process comprising slabbing.fwdarw.hot-rolling.fwdarw.(annealing of hot-rolled strips).fwdarw.acid-pickling of hot-rolled strips.fwdarw.cold-rolling.fwdarw.annealing of cold-rolled strips.fwdarw.acid-pickling of cold-rolled strips, stainless steel is problematic in that its production costs are high. Given that situation, recently, proposed was a technique of omitting a part of those steps of the production process. In particular, hot-rolled steel strips not subjected to the cold-rolling step and the subsequent steps have been widely noticed to reduce the production costs and to shorten the production time.
In general, however, the surface scale of hot-rolled strips of Cr-containing steel such as typically stainless steel is, being different from that of cold-rolled and annealed strips thereof which will be referred to herein under, thick and often has a thickness of a several .mu.m, and, in addition, it is dense and is therefore difficult to remove when compared with the surface scale of carbon steel. For these reasons, therefore, the surface scale of hot-rolled, Cr-containing steel strips is generally removed through pre-treatment of dipping the strips in a sulfuric acid-containing tank followed by acid-pickling finish-treatment of further dipping them in a mixed acid of nitric acid and hydrofluoric acid, for example, according to the technique disclosed in Stainless Steel Handbook (3rd Ed., published by Nikkan Kogyo Newspaper Co. in 1995), page 840. Since the descaling to be effected by acid-cleaning of only such acid-dipping takes a long period of time, mechanical destruction and removal of scale, such as shot-blasting, is often effected prior to acid-dipping for the purpose of shortening the descaling time.
To anneal cold-rolled stainless steel strips, generally employed is a method of treating the strips in a strong reducing atmosphere (blight annealing) or a method of treating them in a combustible atmosphere. The surface film to be formed as a result of the former annealing treatment is extremely thin, and the strips thus treated may have a good surface gloss as they are. However, scale is formed on the strips as annealed by the latter treatment, and it has a negative influence on the corrosion resistance and will gall the die of a mold through which the sheets are press-molded. Therefore, the strips annealed in the latter method must be subjected to acid-pickling. Prior to acid-pickling, the strips are pre-treated by salting them in an alkali salt melt consisting essentially of NaOH and Na.sub.2 NO.sub.3 or by electrolyzing them in a neutral salt solution of Na.sub.2 SO.sub.4, NaNO.sub.3 or the like, and thereafter dipped in an aqueous solution of sulfuric acid, nitric acid/hydrofluoric acid, nitric acid or the like. Optionally, the strip are then further hydrolyzed. Concrete methods of such acid-cleaning are disclosed in, for example, Japanese Patent Publication (JP-B) Sho-38-12162 and Japanese Patent Application Laid-Open (JP-A) Sho-59-59900.
Recently, for some cold-rolled, ferrite-type stainless steel strips, employed is a method of passing the strips at a high speed through a mixed gas atmosphere comprising inert gas such as N.sub.2 gas and a several % by volume of reducing H.sub.2 gas, for example, in a continuous annealing line (CAL) for low carbon steels. Low carbon steel is not oxidized in that reducing atmosphere. As opposed to this, however, when stainless steel containing Cr that is easily oxidized is treated in that atmosphere, a thin oxide film consisting essentially of Fe and Cr and having a thickness of hundreds .ANG. (angstroms) or so is formed on its surface, as so written in Netsu-syori, Vol. 28, No. 6 (1988), pp. 373-378. Therefore, Cr-containing stainless steel treated in that atmosphere requires descaling. Descaling methods are disclosed in, for example, Japan Patent Application Laid-Open (JP-A) Sho-63-216999 and (JP-A) Hei-1-147100.
However, the conventional acid-pickling method of dipping hot-rolled stainless steel strips in sulfuric acid and in nitric acid/hydrofluoric acid is problematic in that, even when it is combined with mechanical descaling, its descaling ability is poor and its producibility is low, and that, although the scale layer could be removed in the method, a Cr-decreasing layer having been formed below the scale layer could not be dissolved satisfactorily therein, resulting in that the corrosion resistance of the steel strips treated in the method is lower than that of steel strips having the same composition and having been mechanically treated to remove the Cr-decreasing layer therefrom through surface polishing. In a hot-rolled stainless steel strip, the Cr-decreasing layer is formed around its surface just below the scale layer formed thereon. Where the hot-rolled band is then cold-rolled, the Cr-decreasing layer will be drawn to be extremely thin, and will no more have any negative influence on the corrosion resistance of thestrip. However, where the hot-rolled strip is, without being cold-rolled, to be a final product which is directly put into practical use, the Cr-decreasing layer still existing as such causes the reduction in the corrosion resistance of the strip.
On the other hand, mechanical descaling such as shot-blasting is problematic in that it gives work defects of so-called "shot blasted marks" to the surface of steel strips, and the shot blasted marks still remain on the surface even after acid-cleaning to worsen the surface properties of the strips. Further, the shot blasted marks worsen the surface gloss of cold-rolled strips, and in addition, have a negative influence on the surface polishing of hot-rolled strips to thereby increase the production costs and lower the producibility.
Given that situation, the object of the present invention is to solve the problems that have heretofore been inevitable in the prior art of producing Cr-containing steel strips such as typically stainless steel strips, and to provide hot-rolled steel strips which have no Cr-decreasing layer on their surface, which have good corrosion resistance even when they are directly used after having been acid-pickled, which have no shot blasted mark, and which have good surface properties and good surface polishability. The invention also provides an. effective method for producing the hot-rolled steel strips.